1. Field of the Invention
This invention relates to a method for prevention of damage to lengths of pipe during shipment. More particularly, the invention is directed to methods of prevention of damage to steel pipe joints used for well completion during ocean shipment from a source, typically a port serving a steel mill, to a destination such as a port serving an offshore oil platform.
2. Description of the Prior Art
Oil and gas wells drilled in the earth are commonly completed by placement of a tubular "casing" in the well. The casing comprises a continuous assembly of steel pipe sections, referred to hereinafter as "pipe joints". The pipe joints are typically externally threaded at both ends in order to be joined by internally threaded pipe couplings to form a continuous casing through which oil and gas are brought to the surface from a formation in the earth. Commonly the casing is surrounded by a layer of cement serving to consolidate the formation around the casing and preventing collapse of the formation as oil and gas are produced.
Successful production of oil and gas from a cased well requires that the casing retain its integrity, that is, that the casing does not corrode to the point of leakage, crack, or fracture during the period of production of the well, which may be several decades. One significant failure mechanism of well casings involves corrosion due to the presence of hydrogen sulfide, H.sub.2 S. Hydrogen sulfide is found in many formations. Additionally, certain commonly used "drilling muds" employed to lubricate the drill bit and flush cuttings away from the face of the drill bit contain materials which release hydrogen sulfide over time, so that to the extent some of the drilling mud remains in the well it may function as a source of corrosive agents.
To combat corrosion of well casings caused by hydrogen sulfide, whether native in the reservoir itself or introduced as part of the drilling or completion processes, pipe joints manufactured of relatively soft steels such as steels of grades known to the art as L-80 and C-95 are typically employed. These relatively soft steels are corroded relatively less upon exposure to H.sub.2 S than are harder steels that might otherwise be suitable as well casings.
The art is well aware that hardening of such soft steels markedly increases their susceptibility to corrosion, particularly in the presence of H.sub.2 S. Such steels experience substantial work hardening responsive to relatively minimal surface impacts. For example, it was found in one test carried out by the assignee of the present application that simply dropping a single pipe joint from the height of approximately 41/2 feet onto a concrete surface caused sufficient work hardening to render the pipe joint unsatisfactory for service. Therefore, such pipe joints must be protected from impact damage to the maximum degree possible.
Most wells are completed using pipe joints manufactured at steel mills remote from the producing field. Wells in Nigeria, for example, or the North Sea are commonly completed using pipe joints manufactured in Japan or the United States. The pipe joints are shipped by sea from the mills to the wells as needed. Extended ocean voyages are thus involved, often through regions experiencing violent weather, such as the North Atlantic Ocean.
Pipe joints are normally stacked in longitudinal rows in the hold of a ship for shipment. If the pipe joints were simply piled loosely in the hold of the ship, they would roll laterally during normal weather conditions, and violently so during storms. Such unrestrained lateral rolling would allow the pipe joints to impact one another, and would lead rapidly to work hardening of the pipe joints. The art is well aware of the need to provide the maximum possible protection of pipe joints from damage during shipment. Accordingly, the art has sought both to minimize such rolling of the pipe joints during shipment (although it is impractical to provide each pipe joint with a secure individual mounting for shipment) and to reduce damage caused when adjacent pipe joints impact one another.
More particularly, the art has developed practices for minimizing movement of pipe joints during a voyage, and teaches placing circular impact-absorbing bumper members around each individual pipe joint at several locations therealong, to prevent the pipe joints from directly impacting one another in the event some rolling does take place. However, the bumper members taught by the prior art are not wholly satisfactory.
More specifically, molded plastic circular ring bumpers are in common use for preventing contact between pipe joints during shipment. One type of such molded plastic ring bumper is sold under the tradename "Protek". Typically such molded plastic ring bumpers include two "C"-shaped halves pivoted to one another by a riveted pivot pin. A removable pin passing through mating bores in the two halves of the ring bumper opposite the riveted pivot pin secures the two halves together. In order to assemble the ring bumper over a pipe joint, the removable pin is removed and the two halves of the ring bumper are pivoted about the riveted pivot pin, opening the ring. The ring bumper is placed over the pipe joint, reclosed, and the removable pin is inserted in the mating bores, to secure the ring bumper over the pipe joint.
Three such molded plastic ring bumpers are typically placed over each pipe joint prior to loading. Use of the molded plastic ring bumpers provides a modicum of impact absorption should adjacent pipe joints impact one another during shipment. However, such ring bumpers are formed of a relatively heavy, dense, hard plastic material. This material is susceptible to being cracked, fractured or in some cases even shattered when subject to heavy impact loads as may be experienced due to shifting of the pipe joints during rolling of a transport ship experiencing bad weather conditions. Commonly, such molded plastic bumper rings are so badly battered after a single voyage that a large percentage cannot be reused; accordingly the bumper rings are typically discarded after a single voyage, at substantial cost. In some cases, such bumper rings are so badly damaged after a single voyage that they do not remain assembled over the corresponding pipe joint, but simply fall off the pipe joint in fragments. If fragments of the molded plastic bumper rings fall upon stevedores or other workers while the pipe joints are being lifted by a crane during off-loading, serious injury can result, especially given the very dense nature of the plastic used to make the bumper rings.
Use of molded plastic bumper rings to protect pipe joints during shipment also requires that appropriate numbers and sizes of the bumper rings be on hand at the shipping location on the shipment date; if there is any error in the placement or fulfillment of a particular order of such rings corresponding to the pipe joints to be shipped in a particular shipment, any delay in delivery of the order, or any change in the numbers of different sizes of pipe joints to be shipped, the shipment of pipe joints may be significantly delayed. Delay in shipment of the pipe joints can cause delay in well completion, occasioning very significant inconvenience and expense.
Certain of the steel mills producing pipe joints for the oil well completion industry have provided pipe joints encircled with spiral members formed of rubber tubing, referred to as "pigtails". These rubber pigtails provide a modicum of protection against damage caused by impacting of the pipe joints against one another. However, it has been found that the impact loads experienced by pipe joints during ocean voyages are frequently sufficient to pinch and cut the pigtails, such that the pigtails do not prevent the pipe joints from contacting one another and becoming work hardened during oceanic shipment.
Therefore, it is apparent that there exists a need in the art for a method of shipment of pipe joints minimizing impact damage to pipe joints during oceanic shipment. More particularly, the art requires a simple, low cost ring bumper and method of applying the same to pipe joints for protecting the pipe joints from impacting one another during shipment. Additionally, it will be apparent that a need exists in the art for a suitable bumper member for protecting pipe joints from one another during shipment that is not rendered useless after a single voyage, so that it can be economically reused.